Project Summary Vitiligo is the most common skin depigmentation disorder that affects 65-95 million people worldwide. The disease is characterized by a selective loss of epidermal melanocytes leading to loss of skin pigmentation in patches that can be localized to small areas or spread widely. This disease causes not only disfigurement, but also affects the patients? psychological health with respect to self-esteem and quality of life. Current strategies for treatment of vitiligo are only moderately effective at best, are not durable, and are practically and financially burdensome. Vitiligo is a complex disease with intricate interactions among and contributions from the immune system, genetic predisposition, and environmental factors. Nevertheless, vitiligo is fundamentally an autoimmune disease in which CD8+ T cells (also known as cytotoxic T lymphocytes or CTLs) play the key effector role leading to destruction of melanocytes. These skin resident T cells must be tightly controlled, lest they get inadvertently activated to become auto-reactive. This control is achieved through the regulatory T cells (Tregs), which dampen the CTL responses. It is, however, unknown whether the melanocytes and/or melanocyte stem cells themselves play an active role in protecting themselves against a possible CTL attack via inherent self-immunoprotective mechanism(s). Here we propose such a novel mechanism for the first time. Cytotoxic T lymphocyte antigen 4 (CTLA4/CD152) is the most critical player in the negative and homeostatic regulation of T cell proliferation and activation. Intriguingly, we have reported that CTLA4 is expressed at low levels in primary human melanocytes, but not keratinocytes or other normal cell types, which raises the question regarding the function of CTLA4 in melanocytes. We posit that CTLA4 expressed on the surface of melanocytic cells may directly inhibit both CTLs and APCs via interaction with the B7 ligands expressed on both these cell types, which would lead to melanocyte/melanoma cell-mediated CTL inactivation and anergy leading to immunoevasion. Such immunoprotective effect may be an important part of the homeostatic function of CTLA4 by shielding the melanocyte stem cells (MSCs) during the destructive (catagen) phase of the hair follicle cycle. It is tempting to speculate that CTLA4 may be important for the maintenance of the MSC ?immune privilege? (IP) and its loss in MSCs may cause the collapse of IP, which would lead to autoimmune destruction and vitiligo. The overall goal of this project is to delineate a novel mechanism of immunoevasion and survival of the melanocyte stem cells in skin. The specific hypothesis is that CTLA4 expression in melanocyte stem cells in the hair follicle (HF) bulge region determines their survival and immune privilege, and that loss of CTLA4 expression causes loss of MSCs, which leads to vitiligo. Moreover, it is hypothesized that CTLA4 expressed on the melanocytic cells directly inhibits effector T cells and APCs by a B7-mediated reverse inhibitory signaling mechanism. The proposed hypotheses will be addressed in two specific aims. In the first aim, we will characterize CTLA4 as a guardian of the melanocyte stem cell immune privilege in skin. In the second aim, the mechanism of the immunoinhibitory function of CTLA4 expressed by melanocyte stem cells will be delineated.